Laser beam scanning device, image forming apparatus, and laser beam detecting method by the laser beam scanning device

Abstract

A laser beam scanning device is disclosed. The laser beam scanning device includes a diffraction optical element which forms a pattern of a diffraction image including two images extending in a direction corresponding to the sub scanning direction and one image extending in a direction inclined by θ (0<θ<90°) from the direction corresponding to the sub scanning direction by inputting a laser beam which is led to a surface of a photoconductor drum, and a light receiving element which receives the pattern of the diffraction image.

1. A laser beam scanning device which scans a surface to be scanned by using a laser beam, comprising:
 a light source which emits the laser beam; 
 a laser beam deflecting unit which deflects the laser beam emitted from the light source; 
 a scanning optical system which condenses the laser beam deflected by the laser beam deflecting unit onto the surface to be scanned; 
 a diffraction optical element which forms a pattern of a diffraction image by dividing the laser beam deflected by the laser beam deflecting unit into a plurality of laser beams; and 
 a light detector which detects the pattern of the diffraction image, wherein, 
 the pattern of the diffraction image at the light detector includes a plurality of images where intervals between the plural images are different in a first direction corresponding to the main scanning direction at a position in a second direction corresponding to the sub scanning direction at least at a part of the plural images. 
 
   
   
     2. The laser beam scanning device as claimed in  claim 1 , wherein:
 the pattern of the diffraction image includes at least a part of an image extending in the second direction and a part of an image extending in a direction different from the first direction and the second direction, and has rotational symmetry of 180° about an optical axis. 
 
   
   
     3. The laser beam scanning device as claimed in  claim 2 , wherein:
 the pattern of the diffraction image further includes at least a part of another image extending in the second direction. 
 
   
   
     4. The laser beam scanning device as claimed in  claim 1 , wherein:
 the image in the pattern of the diffraction image is a dot string image or a line image. 
 
   
   
     5. The laser beam scanning device as claimed in  claim 4 , wherein:
 the light detector is formed of one or more light receiving elements, and the width of the light detector is greater than the width of the dot string image or the line image in the first direction. 
 
   
   
     6. The laser beam scanning device as claimed in  claim 4 , wherein:
 the length of the light detector in the second direction is different from a value which value equals a distance between dots of the dot string image in the second direction multiplied by an integer. 
 
   
   
     7. The laser beam scanning device as claimed in  claim 1 , wherein:
 when the width in a diffraction angle distribution in the first direction is θ max, an interval between pixels of the diffraction optical element is “p”, a using wavelength is λ, and λ>“p”, the interval “p” is determined to satisfy the following inequality:
   arcsin(λ/ p )>θmax. 
 
 
   
   
     8. The laser beam scanning device as claimed in  claim 1 , wherein:
 the diffraction optical element has a structure in which unit components are periodically arrayed, each unit component is formed of pixels, boundaries among the unit components are smoothly connected, and the maximum diagonal length of the unit component is less than the spot diameter of the laser beam input to the diffraction optical element. 
 
   
   
     9. The laser beam scanning device as claimed in  claim 1 , wherein:
 a turning on timing of the light source is determined so that a high-order diffraction image is not input to the light detector. 
 
   
   
     10. The laser beam scanning device as claimed in  claim 1 , wherein:
 the diffraction optical element has a concave-convex structure formed three dimensionally and forms the pattern of the diffraction image by spatially modulating the phase of the laser beam input to the diffraction optical element. 
 
   
   
     11. The laser beam scanning device as claimed in  claim 1 , wherein:
 when an interval between pixels of the diffraction optical element is “p”, a using wavelength is λ, and the refractive index of a substrate of the diffraction optical element is “n”,the following inequality is satisfied:
   λ/ n>p.    
 
 
   
   
     12. The laser beam scanning device as claimed in  claim 1 , wherein:
 the laser beam is input to the diffraction optical element at a position outside an image forming region on the surface to be scanned where an image is formed, and the light intensity of the laser beam which is led to outside the image forming region is greater than the light intensity of the laser beam which is led to the image forming region. 
 
   
   
     13. The laser beam scanning device as claimed in  claim 1 , wherein:
 the light detector outputs signals which are changed by a time interval between positions where the laser beam passes through the pattern of the diffraction image in the second direction. 
 
   
   
     14. The laser beam scanning device as claimed in  claim 1 , wherein:
 the light detector is located at a position further along an optical path than a position where the laser beam is condensed by a scanning optical system; and 
 a lens is positioned to condense the laser beam from the diffraction optical element so that the position of the light detector is conjugated with the position where the laser beam is condensed. 
 
   
   
     15. The laser beam scanning device as claimed in  claim 1 , further comprising:
 an aperture which controls the width of the laser beam to be input to the light detector in the second direction. 
 
   
   
     16. The laser beam scanning device as claimed in  claim 1 , further comprising:
 a shifting unit which shifts a laser beam condensing position on the surface to be scanned based on a signal output from the light detector. 
 
   
   
     17. The laser beam scanning device as claimed in  claim 16 , wherein:
 the shifting unit obtains a shifting amount and a shifting direction of the laser beam condensing position by using a rise timing and a fall timing of the signal output from the light detector. 
 
   
   
     18. The laser beam scanning device as claimed in  claim 16 , wherein:
 the shifting unit includes a liquid crystal deflection element which deflects the laser beam output from the light source in the second direction corresponding to a driving signal; 
 a driving signal generating circuit which generates a driving signal based on the signal output from the light detector; 
 a posture adjusting unit which adjusts the posture of an optical element disposed between the light source and the laser beam deflecting unit based on the signal output from the light detector; and 
 an image forming start position adjusting unit which adjusts an image forming start position on the surface to be scanned in the sub scanning direction. 
 
   
   
     19. An image forming apparatus, comprising:
 at least one laser beam scanning device which scans a surface to be scanned by using a laser beam; and 
 a transferring device which transfers an image formed on the surface to be scanned by the laser beam scanning device to a recording medium, 
 wherein the laser beam scanning device includes: 
 a light source which emits the laser beam; 
 a laser beam deflecting unit which deflects the laser beam emitted from the light source; 
 a scanning optical system which condenses the laser beam deflected by the laser beam deflecting unit onto the surface to be scanned; 
 a diffraction optical element which forms a pattern of a diffraction image by dividing the laser beam deflected by the laser beam deflecting unit into a plurality of laser beams; and 
 a light detector which detects the pattern of the diffraction image, wherein: 
 the pattern of the diffraction image at the light detector includes a plurality of images where intervals between the plural images are different in a first direction corresponding to the main scanning direction at a position in a second direction corresponding to the sub scanning direction at least at a part of the plural images; and 
 the laser beam deflecting unit is formed of polygon mirrors stacked in the second direction in which each polygon mirror has four surfaces or six surfaces. 
 
   
   
     20. A laser beam detecting method in a laser beam scanning device which scans a surface to be scanned by using a laser beam, the method comprising:
 emitting the laser beam from a light source; 
 deflecting the laser beam emitted from the light source; 
 condensing the deflected laser beam onto the surface to be scanned; 
 forming a pattern of a diffraction image by dividing the deflected laser beam into a plurality of laser beams; and 
 detecting the pattern of the diffraction image, wherein, 
 the pattern of the diffraction image at the light detector includes a plurality of images where intervals between the plural images are different in a first direction corresponding to the main scanning direction at a position in a second direction corresponding to the sub scanning direction at least at a part of the plural images.